Many modern cellular telephones, such as the smart phone sold under the trademark “iPhone” by Apple Computer, Inc, use capacitive touch screens. Touch screens are used to locate the two-dimensional position of an object such as a human finger on the surface of the touch screen. These touch screens are substantially transparent to allow an underlying screen display to be visible to the user, allowing the user to interact intuitively with the images in the screen display. A capacitive touch screen is formed by a grid of rows and columns of conductors in which the rows and columns are separated by a dielectric material. The intersections of the rows and columns represent possible touch positions. A touch screen controller transmits pulses to each of the rows of the grid while measuring resulting signals on each of the columns. The controller identifies a touch position based on small disturbances in capacitance caused by the user's finger touching the surface of the touch screen. However since the disturbances in capacitance are small, nearby electrical circuits such as phone chargers that emit significant electrical interference can corrupt the position determination.